Studying the three Welshborn Samuel Madleys on Latterday Saints for 1861 & 1881 Census, all in the Pontypool corner of Monmouthshire, quite possibly all related.

Oldest is:

England and Wales Census, 1891 for Samuel Madley
Name: Samuel Madley
Event: Census
Event Date: 1891
Gender: Male
Age: 33
Relationship to Head of Household: Head
Birthplace: Varteg, Monmouthshire
Record Type: Household
Registration District: Pontypool
Sub-district: Pontypool
Ecclesiastical Parish: Abersychan
Civil Parish: Trevethin With Pontypool
County: Monmouthshire

SO born abt 1858 so abt Annie's age.
Varteg is a few miles north of ABERSYCHAN ! -> aka ABERSCYAN !!

Here he is in 1901 Census:

England and Wales Census, 1901 for Samuel Madley
Name: Samuel Madley
Event: Census
Event Date: 31 Mar 1901
Gender:
Age: 41
Relationship to Head of Household: Son
Birthplace: Panteg, Monmouthshire
Record Type: Household
Registration District: Chepstow
Sub-district: Chepstow
Ecclesiastical Parish: Newchurch With Devanden
Civil Parish: Newchurch East
County: Monmouthshire

So like Annie, Samuel is in CHEPSTOW AREA albeit with his mother at 1901 Census. Newchurch is a small village up valley from Chepstow and more Magor area where Sam Jr was born.

Still no clue from this if Sam & Annie are actually linked yet/still ?

Next is;

England and Wales Census, 1911 for Samuel Madley
Name: Samuel Madley
Event: Census
Event Date: 1911
Gender: Male
Age: 52
Birthplace: Pontypool, Monmouthshire
Record Type: Household
Registration District: Monmouth
Sub-district: Coleford
Parish: West Dean
County: Monmouthshire

Which I think suggests same Household including Annie and Samuel Jr posted earlier;

1911 Wales, Monmouthshire, West Dean Coleford
living in 4 rooms, Berry Hill, Coleford

MADLEY
Samuel - 52 General Labourer, born Mon. Pontypool
Annie - 56 - married 25 years, 5 children born alive, 5 living, born Glos. West Dean
Samuel - 20 - Coal Miner Hewer, born Mon. Aberchan
etc etc

(note both Sam's birthplaces tie in. It is highly likely the men are colliery workers considering their places of birth/residence thro the Census').

BUT STILL NO MARRIAGE RECORD !?

I have searched all thro Latterday Saints site without luck for a marriage 1888 - 1901. Nothing even similar.

As per OP, Annie dies a Madley, this is her only possible burial on FoD site even ignoring quoted age:

Record_ID: 263679
Entry_Number:
Year: 1929
Month: Aug
Day: 21
Surname: MADLEY
Forenames: Ann
Residence: Five Acres
Age_at_death: 71
Officiating_Minister: A Hoyle vicar
Event: Burial
Cause_of_death:
Memoranda:
Notes:
Register_Reference: P82 IN 1/16
Page_No: 45
Parish_Chapel: Christchurch
Soundex: M340

Yet wrt age, if 71 in 1929 she's born abt 1858 which is very close, and correct area.

Only Sam Madley Burial is:
Record_ID: 4400
Entry_Number: 643
Year: 1912
Month: Mar
Day: 3
Surname: MADLEY
Forenames: Samuel
Residence: Berry Hill
Age_at_death: 50
Officiating_Minister: C Barnes vicar
Event: Burial
Cause_of_death:
Memoranda:
Notes:
Register_Reference: P82 IN 1/15
Page_No: 81
Parish_Chapel: Christchurch
Soundex: M340

Again right area, same Chapel; do Register References infer adjacent plots ?.
This Samuel was born abt 1862, again this ties in nicely.

SO.... sorry despite my best efforts cannot find a Marriage Record.
I suspect this would be a Registry Office marriage so outside the Parish Registers? etc.
Searching the Glos Non Conformist Records I can only find a record for Leonard Medley (as seen in Annie's 1901 Census household earlier):

1936 Record of Baptism(no specific date!?) of Lloyd Leonard Madley
Father Leonard Madley,miner. Mother Rose Hannah nee LLOYD.
Zion Chapel, Berry Hill, West Dean.

http://ww3.gloucestershire.gov.uk/genealogy/Results.aspx
1936

Their Marriage of this site:

Record_ID: 1014732
Entry_Number: 26
Year: 1935
Month: Jun
Day: 8
Grooms_Surname: MADLEY
Grooms_Forenames: Leonard
Grooms_Age: 40
Groom_Condition: Bachelor
Grooms_Occupation: Collier
Grooms_Residence: Berry Hill
Grooms_Fathers_Surname: Madley
Grooms_Fathers_Forenames: Samuel
Grooms_Fathers_Occupation: Collier
Brides_Surname: LLOYD
Brides_Forenames: Rose Hannah
Brides_Age: 40
Brides_Condition: Spinster
Brides_Occupation:
Brides_Residence: Berry Hill
Brides_Fathers_Surname: Lloyd
Brides_Fathers_Forenames: William
Brides_Fathers_Occupation: Collier
Licence_or_Banns: Certificate
Date_of_Banns:
Signature_or_Mark: Both Sign
Witness_1: M.G. Short
Witness_2: I.D. Constable
Other_Witnesses:
Officiating_Minister: Frank E. Yeomans
Event: Marriage
Memoranda: Bride's father is deceased
Notes:
Register_Reference: D6529 1/1
Page_Number: 13
Parish_Chapel: Berry Hill Methodist
Soundex_Groom: M340
Soundex_Bride: L300

First find is this, relevant ?:

England and Wales Census, 1881 for George William Harris
Name: George William Harris
Age: 3
Gender: Male
Birth Year: 1878
Birthplace: West Dean, Gloucestershire, England
Relationship to Head of Household: Grand Son
Marital Condition: Single
Profession/Occupation:
Address: Five Acres
Census Place: West Dean (Gloucs), Monmouthshire, England
Disability:
Record Type: Household
Family History Library Film: 1342259
The National Archives Reference: RG11
Piece/Folio: 5225 / 52
Page Number: 4
Household Gender Age
Morgan Senr Watkins M 50
Rose Anna Watkins F 53
Moses Watkins M 17
George William Harris M 3

So a William Harris b1877 is grandson of Rose Anna Watkins b1828...??
Rose ANNA is born Lydbrook (nr Christchurch as per OP), wife of Morgan (a stone quarryman born Monmouth).
https://familysearch.org/pal:/MM9.1.1/XQ3Z-ZT4
All seems relevant..

However also find:

England and Wales Census, 1881 for William Harris
Name: William Harris
Age: 4
Gender: Male
Birth Year: 1877
Birthplace: Blaenavon, Monmouthshire, England
Relationship to Head of Household: Son
Marital Condition: Single
Profession/Occupation: Scholar
Address: 18 William St
Census Place: Llanover, Monmouthshire, England
Disability:
Record Type: Household
Family History Library Film: 1342261
The National Archives Reference: RG11
Piece/Folio: 5237 / 116
Page Number: 30
Household Gender Age
Parent Mary A Harris F 40
Margaret Harris F 13
William Harris M 4
Wm Williams M 63

So another William, son of Mary A (Anne??) Harris:

Name: Mary A Harris is a WIDOW, but born 1841 Garndyrris Monmouthshire, so ?
Prob NOT.

THEN FIND:

England and Wales Census, 1881 for William Harris
Name: William Harris
Age: 3
Gender: Male
Birth Year: 1878
Birthplace: Gloucester, Gloucestershire, England
Relationship to Head of Household: Son
Marital Condition: Single
Profession/Occupation:
Address: 2 Richmond Pde
Census Place: Chepstow, Monmouthshire, England
Disability:
Record Type: Household
Family History Library Film: 1342258
The National Archives Reference: RG11
Piece/Folio: 5219 / 59
Page Number: 43
Household Gender Age
Parent Willm Harris M 25
Parent Annie Harris F 26
William Harris M 3
Bertie Harris M 2

https://familysearch.org/pal:/MM9.1.1/XQ3C-N4L

SO MOTHER is

Annie Harris
Age: 26
Gender: Female
Birth Year: 1855
Birthplace: Masterworth, Gloucestershire, England
Relationship to Head of Household: Wife
Marital Condition: Married

Correct age, but Masterworth ??
Cannot find this place, presume is Minsterworth, Glos, upriver from Chepstow etc.

Father:
Willm Harris
Age: 25
Gender: Male
Birth Year: 1856
Birthplace: Chepstow, Monmouthshire, England
Relationship to Head of Household: Head
Marital Condition: Married
Profession/Occupation: Coach Builder Coach Mkr

Family ages suggest this is indeed the marriage as posted above:
Surname First name(s) District Vol Page
Marriages Dec 1875 (>99%)
HARRIS William Monmouth 11a 73
WATKINS Annie Monmouth 11a 73

Not on FoD site, perhaps Non Conformist or Registry Office ?

Try Glos BMD as includes Reg Office Marriages:

Groom Surname Groom Forename Bride Surname Bride Forename District Parish Building Year Register Entry
HARRIS William WATKINS Annie Forest of Dean Monmouth Register Office 1875 10 88 http://ww3.gloucestershire.gov.uk/bmd/MarriageSearch.aspx

ONLY Chepstow Wm Harris Baptism on FoD site for 1855 +/- 5yrs:
Record_ID: 206028
Entry_Number: 394
Year: 1855
Month: May
Day: 13
Parents_Surname: HARRIS
Child_Forenames: William Bendoff
Fathers_Forenames: William
Mothers_Forenames: Elizabeth Ann
Mothers_Surname:
Residence: Nelson S[tree]t
Occupation: Smith
Officiating_Minister:
Event: Baptism
Memoranda: s[on] o[f]
Notes: Any abbreviated forenames have been expanded to the full name
Register_Reference: 1851-1861
Page_Number:
Parish_Chapel: Chepstow
Soundex: H620

There are other "possibles" as William and Ann/Hanna are VERY popular names, but this is most likely I think. Now must find when/why Annie becomes a Madley....

Searching FoD PRs for possible death of Willm Harris Snr, after 1881 Census:

Record_ID: 142579
Entry_Number:
Year: 1888
Month: May
Day: 25
Surname: HARRIS
Forenames: William B
Residence: 3 Exmouth Pl Chepstow
Age_at_death: 33
Officiating_Minister:
Event: Burial
Cause_of_death:
Memoranda: b by A J Davies Bapt Minister Chepstow
Notes:
Register_Reference:
Page_No:
Parish_Chapel: St Arvans Monmouth
Soundex: H620

"William B" dies age 33 so born abt 1855, qed?!.
No inquest into early death on FoD site.
No will on FoD site.

Now search Madley/Harris marriage post 1888 ?.
None on Glos BMD (should have looked earlier !!).
None of FoD

BAck to Latterday Saints, refind 1901 Census as earlier:

England and Wales Census, 1901 for Annie Madley
Name: Annie Madley
Event: Census
Event Date: 31 Mar 1901
Gender:
Age: 45
Relationship to Head of Household: Mother
Birthplace: West Dean, Gloucestershire
Record Type: Household
Registration District: Monmouth
Sub-district: Coleford
Ecclesiastical Parish: Christ Church
Civil Parish: West Dean (Gloucs)
County: Monmouthshire

BUT if she is STILL married, why not shown as "WIFE" ?.
Presumably because husband Samuel? has left household or is dead ?? by 1901

Surname First name(s) District Vol Page

Marriages Mar 1845 (>99%)
LANGLEY Matthew Chepstow 26 51
LAWRENCE Clara Chepstow 26 51
MADLEY Samuel Chepstow 26 51
Mole Fanny Chepstow 26 51
MORGAN Lewis Chepstow 26 51
SHEPHERD Harriet Chepstow 26 51

Marriages Mar 1855 (>99%)
Madley Samuel Newport M 11a 177
MAY Anne Newport M 11a 177

(The only Sam/Ann in this list but too early if "married 25 yrs in 1911" etc)

Marriages Sep 1905 (>99%)
Madley Samuel Pontypool 11a 255
POWELL Florence Gertrude Pontypool 11a 255
REES David Pontypool 11a 255
WISE Florence Pontypool 11a 255

Marriages Dec 1919 (>99%)
Ennis Ralph Latham Monmouth 11a 57
Latham Mary M Ennis Monmouth 11a 57
Madley Samuel M Roberts Monmouth 11a 57
Roberts Lily M Madley Monmouth 11a 57
Of the above I can only find the last 1919 marriage on this FoD records, not surprisingly.

Searching for Harris/Watkins marriage off FreeBDM for all UK gives the following ;

Surname First name(s) District Vol Page
Marriages Dec 1875 (>99%)

GODWIN Emma Monmouth 11a 73
HARRIS William Monmouth 11a 73
PULLEN Caleb Monmouth 11a 73
WATKINS Annie Monmouth 11a 73

Surname First name(s) District Vol Page
Marriages Mar 1881 (>99%)
HARPER Mary Monmouth 11a 41
HARRIS George Hatfield Monmouth 11a 41
HARRIS James Monmouth 11a 41
WATKINS Ann Monmouth 11a 41

I've always believed that when a page record off FreeBMD lists more than one couple, as shown above, then each married couple are shown together, altho I'd be gratefull for confirmation of this. Certainly that looks the case from these, if indeed it is James Harris that our Ann marries ?

Assuming she's born c1855 then this is probably? her first marriage, so now looking for marriage of Ann HARRIS to Sam Madley ? However from FreeBMD I cannot find any Marriages that may fit this, including looking for Hannah as this is a common variation of Ann.

Sorry.
There are an awful lot of these names in the Glam/Mon areas so a difficult search, including several Samuels and Anns etc etc.
The Latterday Saints site also carries many "possibles" but nothing certain, yet...

I presume this is our Annie ?.

England and Wales Census, 1901 for Annie Madley
Name: Annie Madley
Event: Census
Event Date: 31 Mar 1901
Gender:
Age: 45
Relationship to Head of Household: Mother
Birthplace: West Dean, Gloucestershire
Record Type: Household
Registration District: Monmouth
Sub-district: Coleford
Ecclesiastical Parish: Christ Church
Civil Parish: West Dean (Gloucs)
County: Monmouthshire

https://familysearch.org/pal:/MM9.1.1/X9YM-CPJ

Sadly no household shown, I presume this correlates to the 1901 Wales Census as MP Griffiths' post above, and I assume she is the mother of William Harris head of that household ?

living in 4 rooms, Berry Hill, Coleford

MADLEY

Samuel - 52 General Labourer, born Mon. Pontypool
Annie - 56 - married 25 years, 5 children born alive, 5 living, born Glos. West Dean
Samuel - 20 - Coal Miner Hewer, born Mon. Aberchan
Nellie - 18 - General Servant, born Glos. West Dean
Leonard - 16 - belt boy, underground coal mine, born Mon. Newchurch East
Gladys - 13 - School - born ditto
Ivor MADLEY, Grandson, 2 - boy Glos. West Dean

1901 Census, Monmouthshire, West Dean
Berry Hill

William HARRIS, head - Wdr - 24, Driver under ground (horse) - born Glos. West Dean
James HARRIS, brother - 18 - Tram filler underground, born ditto
Samuel MADLEY - nephew - 10 - born Aberscyon, Glam
Sarah MADLEY - niece 8- born West Dean Glos
Nellie MADLEY - niece - 12 - born West Dean Glos
Annie MADLEY - mother - m - 45 Housekeeper at home - born West Dean Glos
Hannah WATKINS - grandmother - wid - 70 - living on own means - born West Dean Glos
Leonard MADLEY - nephew - 5 - born Aberscyon, Glam

I know this message was from sometime ago, but I am hoping that you will get to see this. I was googling the name Madley, and I recognised the names Samuel and Annie Madley. Annie is also my Great, Great Grandmother, so that may make us 2nd cousins?.

If you did want to get in touch my email is jd7171 AT yahoo.com.

On the Madley side of my family I have only got back to the previous generation of James Madley, if you have any further info that would be great.

JD

Possible on FreeBMD?

Marriages Dec 1875
GODWIN Emma Monmouth 11a 73
HARRIS William Monmouth 11a 73
PULLEN Caleb Monmouth 11a 73
WATKINS Annie Monmouth 11a 73